Transition Metal Phosphate-Based Electrode Materials for Lithium Batteries and their Synthesis
Since Goodenough pointed out the value of lithium ion reversible iron phosphate-based electrodes for use in lithium and lithium-ion batteries (J. Electrochemical Society, vol. 144, No. 4, pp. 1188-1194 and U.S. Pat. Nos. 5,810,382; 6,391,493 B1 and 6,514,640 B1) several groups have developed synthesis processes for making lithiated iron phosphates of the ordered-olivine, modified olivine or rhombohedral nasicon structures and other chemical analogs containing transition metals other that iron.
Until now most processes and materials described in the art to manufacture electrochemically active phosphate-based electrodes for use in battery applications are based on solid state reactions obtained with iron+2 precursors intimately mixed with lithium and phosphate containing chemicals that are used individually or as a combination thereof. Iron+2 oxalate and acetate are the more frequently used starting materials for syntheses carried out under an inert or partially reducing atmosphere to avoid transition metal oxidation to a higher level, e.g. Fe+3 for example (see Sony PCT WO 00/60680A1 and Sony PCT WO 00/60679 A1). LiFePO4 active cathode materials with improved electrochemical performance were also obtained using C introduced as an organic precursor during material synthesis (Canadian Application No. 2,307,119, laid-open date Oct. 30, 2000). Addition of carbon powder or C-coating to LiFePO4 increases powder electronic conductivity, normally in the range of 10-9-10-10 Scm−1 for pure LiFePO4 at ambient temperature. More recently, solid-state syntheses of LiFePO4 obtained from Fe+3 precursors such as Fe2O3 or FePO4 have been described. These syntheses use reducing gases or precursors (PTC/CA2001/001350 published as WO 02/27824 and PTC/CA2001/001349 published as WO 02/27823) or are carried out by direct reduction (so-called carbothermic reduction) of mixed raw chemicals with dispersed C powder (Valence, PCT WO 01/54212 A1).
All of these solid-state synthesis reaction ways require relatively long reaction time (several hours) and intimate mechanical dispersion of reactants since the synthesis and/or particle growth in the solid state are characterized by relatively slow diffusion coefficients. Furthermore, particle size, growth, and particle size distribution of the final electrode material are somewhat difficult to control from chemical precursors particle dimensions or in view of the reactive-sintering process, partially suppressed by the presence of dispersed or coated carbon on reacting materials.
Recent attempts to grow pure or doped LiFePO4 in solid state and at high temperature, for example 850° C., have led to iron phosphate with 20 micron single grain sized, intimately mixed with iron phosphide impurities and with elemental C thus making intrinsic conductivity evaluation difficult (Electrochemical and Solid-State Letters, 6, (12), A278-A282, 2003).
None of the previously demonstrated synthesis procedures to make LiFePO4, doped or partially substituted LiFePO4 and transition metal phosphate-based analogs as electrode materials, contemplate a direct molten state phase process in which a liquid, phosphate-containing phase is used to achieve synthesis, doping or simply to melt and prepare electrochemically active lithiated or partially lithiated transition metal phosphate-based electrode materials, especially phosphate-based materials made of iron, manganese or their mixtures obtained in a dense form as a result of a melting/cooling process, optionally comprising one or more synthesis, doping or partial substitution steps.
In fact most known synthesis work on phosphates for use as electrode material suggest working at low temperature to avoid rapid particle growth in the solid state and partial decomposition of the iron phosphate under reducing conditions as such or irreversible decomposition of the precursor chemical at too high a temperature.
Metal Phosphates Preparation by Melting Process
Although inorganic phosphates, pyrophosphates or phosphorous pentoxide have been used with iron oxide and other oxides, to melt and stabilize by vitrification, hazardous metal wastes such as alkali and alkaline earth radioactive elements (U.S. Pat. No. 5,750,824) the chemical formulation of the melt obtained at a temperature in the range of 1100-1200° C., is variable with both Fe+2 and Fe+3 being present. The purpose was indeed to obtain a stable vitreous composition and not a specific formulation and structure that are appropriate for electrochemical activity, i.e. capable of high reversible lithium-ion insertion-desinsertion.
Additional literature on ferric-ferrous or Mn+2—Mn+3 ratios observed in sodium oxide-phosphorus pentoxide melts at lower temperature, for example 800° C., is also found in Physics and Chemistry of Glasses (1974), 15(5), 113-5. (Ferric-ferrous ratio in sodium oxide-phosphorus pentoxide melts. Yokokawa, Toshio; Tamura, Seiichi; Sato, Seichi; Niwa, Kichizo. Dep. Chem., Hokkaido Univ., Sapporo, Japan. Physics and Chemistry of Glasses (1974), 15(5), 113-15.)
A Russian publication describes the growth of LiCoPO4 crystals in air from LiCl—KCl-based melts containing lithium pyrophosphate in order to make X-ray diffraction studies, but no mention or suggestion is made as to the use of melts in a process to prepare electrochemically active lithium-ion inserting phosphate cathodes containing air sensitive iron for use in lithium-ion batteries. Synthesis and x-ray diffraction study of the lithium cobalt double orthophosphate LiCoPO4. Apinitis, S.; Sedmalis, U. Rizh. Tekhnol. Univ., Riga, USSR. Latvijas PSR Zinatnu Akademijas Vestis, Kimijas Serija (1990), (3), 283-4. Another work by Russian authors describes crystal growth from melt of M+3 (including isovalent and heterovalent cations) phosphates for use as superionic conductors including ferric phosphate of the formula Li3Fe2(PO4)3. Nowhere it is shown or even suggested that such material can be electrochemically active as an electrode material, furthermore, their formulations including isovalent metals are not adapted for such use. Furthermore, these phosphate containing materials are fully oxidized and of no use in a lithium-ion battery normally assembled in discharged state (with the transition metal in its lower oxidation state and the reversible lithium-ions present in the electrode after material synthesis). Synthesis and growth of superionic conductor crystals Li3M2(PO4)3 (M=Fe3+, Cr3+, Sc3+). Bykov, A. B.; Demyanets, L. N.; Doronin, S, N.; Ivanov-Shits, A. K.; MeI'nikov, O. K.; Timofeeva, V. A.; Sevast'yanov, B. K.; Chirkin, A. P. Inst. Kristallogr., USSR. Kristallografiya (1987), 32(6), 1515-19.
None of the previous art teaches how to make a lithiated phosphate electrode using a simple and rapid process in which phosphate cathode formulations are prepared in the molten state and cooled in order to obtain a solid cathode material having electrochemical properties that are optimized for use in lithium batteries, especially lithium-ion batteries (synthesis in the discharged or partially discharged state). In fact, previous art on phosphate-based cathode materials suggests that as low a temperature as possible (450-750° C.) is better to achieve good electrochemically active formulation and stoichiometry, for example: LiFePO4 formulation with adequate particle size and optimal electrochemical activity, while avoiding total iron reduction to Fe° or simple thermal decomposition of the iron or other metal phosphate to oxide and P2O5 or to iron phosphides at temperature higher than 850-950° C. In fact, the melting of pure lithiated phosphates, not to say electrochemically active ones, without partial or total decomposition was not expected or described; neither, a fortiori, a process combining chemical synthesis and phosphate cathode formulation melt.